The present invention relates to a vertical mill for such material as cement raw materials, clinker, coal, or the like, provided with a separator.
Generally, various vertical mills have been devised heretofore in order to grind a feed material such as cement raw materials or clinker, and to classify the powdered or granular material of the desirable particle size range from the ground material in one operation. For example, a conventional vertical mill comprises a grinder portion disposed at the bottom of a vertical cylindrical casing, and a separator means positioned above the grinder portion. The separator means has a shaft arranged to be rotatable along the center axis of the cylindrical casing. An impeller is mounted to the lower end of the shaft so that it may be rotated with the shaft.
In such a conventional mill, the ground material is conveyed by an upward moving fluid such as hot gas from the grinder portion to the separator means, and the powdered or granular material of a desirable particle size range is sorted out by the centrifugal force of a rotating impeller. Then, the finer particles are pneumatically conveyed to a suitable extraction conduit which is mounted on the top of the casing, and the coarser particles are radiated outward from the separator means.
However, in such a conventional vertical mill, these coarse particles rejoin the ground material being conveyed from the grinder portion to the separator means, and are returned into the separator means once more, without being conveyed back to the grinder portion. Therefore, as these coarse particles are not reground, the grinding efficiency in this type of mill is low.
An approach to such a problem is to provide a separator means which classifies the ground material by two forces i.e. by the fluid resistance of a vortex flow and by centrifugal force, and to partition this separator means and the grinder portion by a funnel-type inner cone in order to prevent the classified coarse particles from re-joining the ground material being conveyed upward and to positively return the classified coarse particles into the grinder portion, which improves the classifying the grinding efficiencies. However, in large-sized vertical mills, it is normally difficult to generate in a separator chamber an ideal horizontal vortex flow having the required gradient, which is indispensable for ideal classification. Therefore, it has been proposed to mount a plurality of adjusting plates for vortex flow on a rotary disk in the separator means so as to allow the gradient of the vortex flow to be adjusted precisely, and to divide the separator chamber into a plurality of separator chambers by at least one dashboard mounted to the adjusting plates to restrain the fluctuation or displacement of the longituditional component of velocity of the vortex flow.
However, as this kind of vertical mill has a structure where the fluid flow is spirally removed from the grinder portion up to the separator means, it is difficult to generate the ideal horizontal vortex flow in the separator means, even after the above-mentioned improvement. Further, the separator means which generates the vortex flow as mentioned above accelerates the revolution of the rising flow, which increases pressure loss. Additionally cyclone effect makes the ground material which has once been blown up with the rising flow to fall down along the inner wall of the casing, where the material is again blown up with the rising flow so that, accordingly energy is wasted.